from abc import ABC, abstractmethod

from flask import current_app, g, request

from .constants import ReplicaType, PolicyType
from .utils import get_all_subclasses


READONLY_METHODS = {'GET', 'HEAD'}


class RoutingPolicy(ABC):
    """Base policy class for requests routing between replicated db's."""
    type: PolicyType

    @classmethod
    def set_replica(cls):
        """Main method for choosing replica type and save it in g."""
        view_function = current_app.view_functions.get(request.endpoint)
        # view function has 'set_replica' if it was used with set_replica decorator
        replica_type = getattr(view_function, 'set_replica', None)
        if replica_type is not None:
            if replica_type not in ReplicaType:
                raise ValueError(f"Wrong replica type {replica_type} was given.")
        else:
            replica_type = cls.get_replica()
        g._replica_type = replica_type

    @staticmethod
    @abstractmethod
    def get_replica() -> ReplicaType:
        """Return replica type for specific policy."""
        pass


class DefaultMasterPolicy(RoutingPolicy):
    """Policy for using master as default replica."""
    type = PolicyType.DEFAULT_MASTER

    @staticmethod
    def get_replica() -> ReplicaType:
        return ReplicaType.MASTER


class DefaultSlavePolicy(RoutingPolicy):
    """Policy for using slave as default replica."""
    type = PolicyType.DEFAULT_SLAVE

    @staticmethod
    def get_replica() -> ReplicaType:
        return ReplicaType.SLAVE


class RequestTypePolicy(RoutingPolicy):
    """Policy for using replica depending on request type:
    slave - for read only requests, master - for other.
    """
    type = PolicyType.REQUEST_TYPE

    @staticmethod
    def get_replica() -> ReplicaType:
        return ReplicaType.SLAVE if request.method in READONLY_METHODS else ReplicaType.MASTER


_POLICIES_ = {c.type: c for c in get_all_subclasses(RoutingPolicy) if c.type is not None}


def get_policy(type: PolicyType) -> RoutingPolicy:
    return _POLICIES_[type]